Use the QScreen of the QWindow as default output

If the Window::setDesiredOutput API was not called for the QWindow, use
QWindow::screen(). This allows assigning QWindows to specific screens using
the plain Qt API.

Passing nullptr to Window::setDesiredOutput explicitly results in nil as
desired output for the layer, which lets the compositor select a screen.

(cherry picked from commit 3c85e2e8)